A mixture of cyclohexanone and cyclohexanol can be employed as an industrial solvent or as an activator in oxidation reactions. It can also be used as an intermediate, for example in the production of adipic acid or nylon 6,6. Where the mixture is mostly or almost entirely cyclohexanone, this can be used in the production of cyclohexanone resins, caprolactam or nylon 6.
Mixtures of cyclohexanone and cyclohexanol are conventionally prepared from phenol by catalytic hydrogenation in a phenol hydrogenation reactor, e.g. using a platinum or a palladium catalyst. The reaction can be carried out in the liquid phase or the vapour phase. [Kirk-Othmer Encyclopedia of Chemical Technology, e.g. 3rd Edition, Vol 7 (1979) p. 410-416; I. Dodgson et al. “A low Cost Phenol to Cyclohexanone Process”, Chemistry & Industry, 18, December 1989, p 830-833; or M. T. Musser “Cyclohexanol and Cyclohexanone”, Ullmann's Encyclopedia of Industrial Chemistry 7th Edition, 2007), (hereafter “Musser”)].
In the preparation of cyclohexanone from phenol, typically cyclohexanol (which can be considered an intermediate product useful for further conversion to cyclohexanone) and various undesirable by-products are formed.
The cyclohexanone is typically recovered by a distillation process as a mixture of cyclohexanone and cyclohexanol (usually ≧90 wt. % cyclohexanone) or as an essentially pure product (≧99 wt. %).
A conventional process for the preparation of a mixture of cyclohexanone and cyclohexanol from phenol feedstock involves hydrogenation of phenol with gaseous hydrogen in the presence of platinum or palladium in a hydrogenation reactor, to produce a hydrogenation product stream comprising cyclohexanone, cyclohexanol, phenol and hydrogen; cooling the hydrogenation product stream to a temperature, such that the fraction of phenol by mass in a first gas phase is lower than the fraction of phenol by mass in a first liquid phase; separating the first gas phase from the first liquid phase; returning at least part of the first gas phase to the hydrogenation reactor; heating the first liquid phase; and purification of the first liquid phase by distillation. Such a process is described schematically in FIG. 1. In such a process, cooling is necessary to aid separation of hydrogen from the hydrogenation product stream. Heating of the first liquid phase is needed to prepare it for distillation. Typically the separated first gas phase which contains hydrogen is heated before returning it to the hydrogenation reactor.
A problem in the above described processes is that consumption of steam for heating is high. In particular a large amount of energy is used for heating both the first liquid phase, and the first gas phase. At the same time, cooling is applied, meaning that cooling water is required to transfer heat from the process stream. Both heating and cooling accordingly increase cost and carbon foot print of the process as a whole.
It is therefore an object of the present invention to provide a method for preparing a mixture of cyclohexanone and cyclohexanol, wherein the above drawbacks are overcome or at least alleviated.
The present inventors have found that it is possible to reduce energy consumption and reduce the consumption of cooling water and improve the carbon foot print in a process for converting phenol to a mixture of cyclohexanone and cyclohexanol by hydrogenation by introducing one or more additional in-process heat exchange steps. Specifically, heat transferred from the hydrogenation product stream as it is cooled is transferred to another part of the process stream which requires heating.
Accordingly, the present invention provides a process for continuously preparing a mixture of cyclohexanone and cyclohexanol comprising,                a) hydrogenating phenol with gaseous hydrogen, in the presence of platinum or palladium, in a hydrogenation reactor, to produce a hydrogenation product stream comprising cyclohexanone, cyclohexanol, phenol and hydrogen;        b) cooling the hydrogenation product stream to a temperature such that the fraction of phenol by mass in a first gas phase is lower than the fraction of phenol by mass in a first liquid phase;        c) separating the first gas phase from the first liquid phase;        d) returning at least part of the first gas phase to the hydrogenation reactor;        e) heating the first liquid phase;        f) purifying the first liquid phase by distillation;        characterized in that heat is transferred from the hydrogenation product stream in step b) to another part of the process by means of in-process heat exchange.        